NASIRIYAH, Iraq — U.S. Marines maintained a tenuous hold on two key bridges on Tuesday after pushing Iraqi forces from a hospital building. The Marines endured three days of gunfire from Iraqi forces and waves of Iraqis, some in civilian clothes, continued to fire at Marine positions in an effort to deny the Americans control of the highway leading to Baghdad.

The highway traverses two waterways — the Euphrates River and the nearby Saddam Canal — making it a vital supply route for the 1st Marine Division, which crossed over this weekend en route to Baghdad.

The 1st Marine Division left a brigade behind to hold the bridges, but that has turned out to be a tougher job that division commanders may have realized. On Tuesday, a tremendous amount of fire was exchanged as Iraqi soldiers continued to, as one Marine put it, “pester” the Marines and try to close down this road.

Nine Marines died and another 50 were wounded in three days of furious fighting as the Marines tried to secure the bridges in the face of persistent attacks from Iraqi soldiers and militia units, many of whom were in civilian dress.

FIGHT FOR HOSPITAL

On Monday, there was a tremendous fight for control of a hospital off to the southeastern corner of the bridge over the Euphrates. Iraqi soldiers had been using it to take potshots at the Marines, but Tuesday those soldiers were cleared out.

suggesting to some that these troops might have been expecting chemical or biological weapons to be used eventually.

The Marines also found military uniforms tossed on the floor, indicating that a number of Iraqi soldiers had changed into civilian clothes.

It’s hard to tell who the enemy is. There are civilians on the street here walking around and it is often difficult for the Marines to return fire because there are women and children in the foreground.

Several dozen patients and doctors were in the hospital when the Marines cleared it of Iraqi soldiers. Marines said they believe the Iraqis used the hospital as a defensive position because they assumed the Americans would not fire on it.

There was still firing around the two bridges Tuesday night, with the Marines using earthen berms for protection.

The Iraqis are out in relatively open territory in an urban area near the bridge, and whenever they can determine their location, the Marines are calling in artillery strikes. The bridges are in American hands. For now.

(Kerry Sanders is an NBC News correspondent traveling with the First Marine Division in Iraq. )